In the mid-1970's, the United States Department of Health estimated that approximately 56 visits per year are made to medical offices for X-ray examination, for every 100 persons in the United States. Since that time, it is generally agreed that the diagnostic use of X-rays has increased dramatically. The majority of the X-ray procedures performed in the United States are interpreted by a radiologist who reads the transparent film X-ray prints by holding the print adjacent to an intense source of illumination. The X-rays themselves may be of either conventional procedures such as the chest or the skeleton, or may be more specialized as in the case of mammograms, computed tomography or ultrasound/sonograms. Depending upon the procedure, the transparent film X-ray prints may vary in size from 8.times.10 inches (or less) in size up to 14.times.17 or more. Moreover, two to ten or more prints are typically made to document a given procedure. These prints will then be analyzed by comparing them to prints made during previous procedures, to aid the radiologist in spotting deviations in the patient's condition.
It has been generally recognized that the observation of subtle detail in a radiographic film is best done with the aid of very bright illumination from a source of white light on a device known in the field of radiology as a light box. Large medical institutions may use motorized X-ray viewers, sometimes known as alternators, to efficiently view large numbers of X-ray or other transparent prints. As white light is the desired illumination for X-rays, most light boxes employ a large number of fluorescent lamps, such as fluorescent tubes, arranged in a parallel fashion, and rated by "daylight" emission.
A light box of the type commonly employed in hospitals and medical centers around the world is illustrated, in FIG. 1, generally 10, in exploded perspective view. FIG. 1 illustrates a typical light box 10 comprising a light source housing 12 having an open proximal end 12a and a closed distal end 12b. The light source housing 12 may include a backreflector or series of baffles in the distal end 12b to re-direct stray light towards the proximal end 12a of the housing, where X-ray prints will be read. Disposed in the housing 12 is an array of light bulbs or fluorescent tubes 14, driven by conventional or electronic ballasts. The array is connected to a source of electricity, such a wall outlet, to provide energy to illuminate said tubes. Disposed at the proximal end 12a of the box, i.e., the open end, is a translucent light diffuser 16 adapted to diffuse light generated by the light array 14. A diffuser 16 provides a uniform distribution of light across the entire proximal end 12a of the housing 12. This is critical since the average institutional light box or alternator is a wall-sized device having dimensions of four feet by eight feet or more. The diffuser 16 can also be adapted to function as the stage upon which the transparent film print is disposed for analysis.
Further, an article entitled Spatial Modulation Transfer in the Human Eye, by Van Nes and Bauman, Journal of the Optical Society of America, Vol. 57, pp 401-406, 1967 published the results of visual experimentation which proved that the perception of a small contrast in a transparent film, i.e., the relative change in luminance for two regions having a small difference in film density or transmittance, improves as the average luminance is increased up to a luminance of 100 cd/m.sup.2. A typical alternator might have a luminance of approximately 2600 cd/m.sup.2. This will yield a minimum luminance of 8.22 cd/m.sup.2 and a maximum luminance of 1640 cd/m.sup.2 when viewing a film with a minimum film density of 0.2 and a maximum film density of 2.5.
A major limitation of the human eye for viewing images, such as X-ray prints, is related to the eye's adaptation to average luminance. It has been found that the eye's ability to detect small contrast in images is best when the average luminance of the region of the image containing the contrasting object is equal to the average luminance in the surrounding film. See Schreiber, W. F., Fundamentals of Electronic Imaging Systems, Springer-Verlag, Berlin, 1986. Accordingly, if a film with a high density and a low luminance is surrounded by a high luminance background, the response of the eye to contrast in the film is far inferior to the eye's response when the surrounding area has the same luminance as the average luminance of the film. The physiological reasons for this phenomenon have been explained as originating, at least in part, from light scattering within the optics of the eye, as is fully explained in Diffusion of the human retina and quality of the optics of the eye on the fovea and the peripheral retina, Vision Research, 19: 907-911 (1979). Moreover, the deleterious effects of this phenomenon upon the field of radiology has been well documented. See, Changes in lesion detection caused by light adaptation in retinal photoreceptors, Invest. Rad., 17: 394-401 (1982); and Influence of ambient light on the visual sensitometric properties of, and detail perception on, a radiograph, Proceedings SPIE, 273: 57-62, (1981).
Accordingly, the wall-sized light boxes or alternators are generally located in rooms having dim ambient lighting since room light can scatter off the X-ray film print itself, further degrading the quality of the film analysis. Unfortunately, radiographs are produced in varying sizes which are much smaller than the wall sized alternators. This of course produces sub-optimal viewing for the physiological reasons noted above. Numerous solutions have been proposed based upon simple mechanical devices to shutter the alternator in areas not covered by exposed film. Some of these proposed improvements are set forth in A self-masking film illumination system, British Journal of Radiology, 47: 283, (1974); and Automatic view box, American Journal of Roentgenography, 106: 218, (1969). Despite the seeming simplicity of the problem, none of these devices have satisfied the needs of the field, due primarily to poor reliability, inability to effectively mask stray or deleterious light and difficulty in fine adjustment of the mechanical systems for smaller X-ray prints. Ease of operation is a major factor in the utility of the device, given the large number of prints analyzed by the average radiologist each day. Accordingly, radiologists and other medical professionals have been forced to resort to relatively primitive methods of enhancing the viewing of X-ray prints. See Use of the cupped hand for improving viewing of radiographs, Radiology, 143: 563, (1982).
These and other limitations of the prior art are obviated by the invention disclosed and claimed herein.